DAC8822_07 Datasheet, PDF (15/24 Pages) Burr-Brown (TI) – 16-Bit, Dual, Parallel Input, Multiplying Digital-to-Analog Converter

DAC8822
www.ti.com
SBAS390A – DECEMBER 2006 – REVISED MARCH 2007
THEORY OF OPERATION
The DAC8822 is a multiplying, dual-channel, current output, 16-bit DAC. The architecture, illustrated in
Figure 40, is an R-2R ladder configuration with the three MSBs segmented. Each 2R leg of the ladder is either
switched to GND or to the I_OUT terminal. The I_OUT terminal of the DAC is held at a virtual GND potential by the
use of an external I/V converter op amp. The R-2R ladder is connected to an external reference input (V_REF) that
determines the DAC full-scale output current. The R-2R ladder presents a code-independent load impedance to
the external reference of 5kΩ ± 25%. The external reference voltage can vary in a range of –18V to +18V, thus
providing bipolar I_OUT current operation. By using an external I/V converter op amp and the R_FB resistor in the
DAC8822, an output voltage range of –V_REF to +V_REF can be generated.
V_REF
R
R
R
2R 2R 2R 2R 2R 2R 2R
2R
2R
2R
2R 2R
R_FB
I_OUT
GND
Figure 40. Equivalent R-2R DAC Circuit
The DAC output voltage is determined by V_REF and the digital data (D) according to Equation 1:
V_OUT AńB + *V_REF
D
65536
(1)
Each DAC code determines the 2R-leg switch position to either GND or I_OUT. The external I/V converter op amp
noise gain will also change because the DAC output impedance (as seen looking into the I_OUT terminal) changes
versus code. Because of this change in noise gain, the external I/V converter op amp must have a sufficiently
low offset voltage such that the amplifier offset is not modulated by the DAC I_OUT terminal impedance change.
External op amps with large offset voltages can produce INL errors in the transfer function of the DAC8822
because of offset modulation versus DAC code. For best linearity performance of the DAC8822, an op amp
(such as the OPA277) is recommended, as shown in Figure 41. This circuit allows V_REF to swing from –10V to
+10V.
V_DD
U1
V_DD R_OFS R_FB
V_REF
DAC8822 ^IOUT^A/B
GND
+15V
U2
V+
OPA277
V-
-15V
V_OUT
Figure 41. Voltage Output Configuration
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